The invention is directed to a method for the economical production of seamless hot-rolled pipes in continuous pipe rolling mills according to the preamble of patent claim 1. The invention is further directed to a rolling mill according to patent claim 8.
Various methods used for the production of hot-rolled seamless pipe are described in Stahlrohr Handbuch (Vulkan-Verlag, Essen, 12th edition 1995, pages 107-111).
In recent years, it has become increasingly necessary to produce products in a consumer-oriented manner because the consumer countries desire participation through job creation and value added tax. This necessarily entails limitations on the sales market.
Typical products in such cases are, for example, pipes for the energy sector for oil and gas exploration and production.
The current range of dimensions is between approximately 60 mm and 273 mm in diameter and approximately 5 mm to 15 mm in wall thickness.
The required capacity for pipe rolling mills ranges approximately between 100,000 and 250,000 tons per year for these products.
After the round starter material has been heated, seamless hot-rolled pipe is usually manufactured in three process steps:                the solid block is pierced to form the hollow block,        the hollow block is stretched to form the mother pipe, and        the mother pipe is finish-rolled to the hot finished pipe dimensions.        
Owing to the fact that, barring exceptional cases, cross rolling piercing mills are used for the first finishing step and finish rolling is carried out exclusively by means of stretch reducing mills or sizing mills, the rolling mills as a whole are named after the stretch reducing mills that are used.
Rolling mills with a yearly capacity in the above-mentioned range are push bench rolling mills, Assel rolling mills and Diescher rolling mills. In the latter two, cross rolling mills are used for stretching.
Operation of these rolling mills requires a high level of know-how, since it is not easy to produce pipe without external or internal defects. Typical pipe defects are, e.g., small cracks, to some extent having a shallow depth. The risk of defects increases as wall thickness decreases. Therefore, the ratio of diameter to wall thickness is limited. In Assel mills, for example, this ratio is 20:1. In Diescher mills, internal cracks can hardly be avoided, so that the pipes must be reworked.
This qualitative disadvantage and the demanding requirements of the oil and gas industry prohibit the use of these rolling methods for the production of premium products without costly mechanical work on the inner and outer surfaces of the pipe.
For these exacting products, longitudinal rolling has found general acceptance for quality reasons for stretching the hollow block by the continuous pipe rolling process; in longitudinal rolling, the hollow block undergoes a reduction in cross section of up to 75% in up to nine roll stands which are arranged closely one behind the other, which results in a four-fold stretching in length. The reduction in cross section to the mother pipe dimensions required for finish rolling is carried out continuously. A method of this kind is known, for example, from EP 1 764 167 B1.
The possible range of dimensions in the continuous pipe rolling method is approximately 25 mm to 498 mm in outer diameter; this diameter range cannot be covered by an individual rolling mill. Disregarding the furnace for heating the starter material, a current-day continuous pipe rolling mill in its entirety typically has the following composition:                cross rolling mill for piercing with a maximum length of the hollow block between 11 m and 12.5 m,        stretching unit (e.g., 2-roll or 3-roll continuous pipe rolling mill with retained bars) having 5 or 6 stands,        bar circulation of the stretching unit with 5 to 8 bars, bar length of around 20 m, approximately one half thereof as working part for rolling, the other half for bridging the distance between the actual rolling mill and the bar retaining system,        extracting mill comprising 3 stands, each having 3 rolls, for removing the mother pipe from the rolling bar,        reheating furnace,        sizing mill or stretch reducing mill,        cooling bed.        
The rolling bar retaining system has the following purpose:                threading the rolling bar into the hollow block,        pushing the hollow block with rolling bar into the first stand of the rolling mill,        retaining the rolling bar during rolling in such a way that it moves forward at a constant speed below the entry speed of the hollow block into the first stand,        returning the rolling bar to the entry side of the rolling mill after rolling is terminated.        
The rolling bar is then ejected laterally into the bar circulation for cooling and lubrication and a “new” rolling bar is delivered to the rolling bar retaining system on the return side of the bar circulation.
In the known plant, the extracting mill is at a distance of approximately 10 m to 12 m from the end of the final stand of the two-roll or three-roll continuous pipe rolling mill. The extraction of the mother pipe from the rolling bar begins as soon as the tip of the mother pipe enters the first stand of the extracting mill. At this time, part of the mother pipe is still located in the continuous pipe rolling mill. As soon as the mother pipe has exited the extracting mill, the rolling bar is withdrawn. At this time, the tip of the rolling bar is located just in front of the first stand of the extracting mill.
Often, the mother pipes must be reheated prior to finish rolling. There are two reasons for this. First, the temperature varies for different wall thicknesses of the mother pipe. Thin-walled pipes cool much faster than thick-walled pipes. Given the same diameter at the exit of the stretch reducing mill or sizing mill, this influences the cold finished pipe diameter depending on the differing amount of shrinkage. A second reason is that a cooling of the mother pipes below about 600° C. allows a normalizing of the material when subsequently heated again in the reheating furnace to temperatures above Ac3.
Apart from the capability of these known rolling mills to roll premium-quality pipe, plant concepts of this kind have a very high production capacity which ranges from 300,000 to 900,000 tons/year depending on the range of dimensions and on production time.
This method is made particularly economical through the possibility of continuous rolling of multiple lengths, i.e., in accordance with the required pipe length, a hollow block length is used which, when stretched, yields a multiple length which is then severed to the required individual pipe length after finish rolling with a minimum of waste scrap.
However, it is disadvantageous that with an annual capacity of only 100,000 to 250,000 tons for premium product these rolling mills which are designed for high annual capacities and set up at high investment costs cannot be operated efficiently.
EP 1 764 167 B1 provides suggestions for increasing efficiency particularly by doing away with the extraction mill. Owing to a controlled movement of the mandrel bar counter to the rolling direction in the continuous rolling mill, this mandrel bar is substantially removed from the mother pipe at the conclusion of the rolling process so that a separate extraction mill is superfluous.
However, there are practical drawbacks to the described technical steps suggested for complete removal of the rolling bar from the mother pipe (stripping). For example, stripping by means of a stripper—at least in the case of thin wall thicknesses—always entails flaring of the end of the mother pipe which must absolutely be sawed off before the subsequent sizing or stretch reduction. In technological respects, stripping by means of a roller table is also critical because the time during which the process takes place cannot be controlled.
Further, the described steps are still not sufficient for substantial improvement of the efficiency of the process because the investment costs for the rolling mill are still very high in spite of the elimination of the extraction mill. Therefore, additional cost-cutting measures must be taken to increase efficiency.
A method for producing wire, rods, or seamless pipes on a rolling mill is known from EP 1 764 167 B1. To achieve an optimal mode of operation with reduced plant costs, a three-roll continuous rolling mill is used in the main stage for rotary piercing in pipe production and for solid rolling in the production of rods, wire or the like.
Finally, the production of seamless steel pipes in three deformation steps, including piercing in a cross rolling mill, stretching in an Assel mill, continuous rolling mill or other mill, and finish rolling in a stretch reducing mill, is known from EP 1 102 033 A1.